In the field of clinical medical practice, it is required to easily judge the presence or absence of a disease, the degree of progression of the disease, the effect obtained after a treatment, etc. Under these circumstances, as a method for judging them indirectly, detecting a marker whose expression amount changes specifically accompanying the onset or progression of each disease has been proposed, and attempts actually are made to put this into practical use.
Among various diseases, detecting malignant tumors, which are so-called cancers, early and selecting and changing a treatment strategy therefor appropriately are particularly important. Thus, in order to realize indirect judgment by the detection of a marker as described above, various cancer markers (tumor markers) have been reported. Specific examples of the cancer marker include PSA (Prostate-Specific Antigen), CEA (Carcinoembryonic Antigen), CA 19-9 (Carcinoembryonic Antigen 19-9), and CA 72-4 (Carcinoembryonic Antigen 72-4). Furthermore, it is described in Non-Patent Documents 1 and 2 that the expression of miRNAs such as has-mir-15, has-mir-16, miR-143, and miR-145 is downregulated in lymphocytic leukemia, colon cancer, and the like (Non-Patent Documents 1, 2).